Patients with heart problems are often treated by surgical implantation of a cardiac stimulation device (such as an implantable cardioverter-defibrillator, or “ICD”) providing one or more stimulation therapies, such as pacing, cardioversion or defibrillation. Many ICDs provide “dual chamber” therapy—that is, the ICD is capable of sensing activity in an atrium and in a ventricle, and is also capable of delivering stimulation energy to the atrium and/or the ventricle.
ICDs are capable of operating in various modes. One such mode is a DDD mode, in which the ICD senses activity in both the right atrium and right ventricle, and also is capable of delivering stimulation energy to both the right atrium and right ventricle.
In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
ICDs operating in a DDD mode generally function as follows. If an intrinsic atrial depolarization signal (a P-wave) is not detected by the atrial channel before an escape interval times out, a stimulating pulse will be delivered to depolarize the atrium to cause atrial contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrial-ventricular interval (AV interval, also referred to as AV delay), a stimulation pulse is delivered to the ventricle to cause ventricular depolarization. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event.
ICDs are also capable of switching modes, for example, from DDD to VVI, in which the ICD no longer uses atrial activity to control ventricular activity. One scenario in which the device would switch from DDD to VVI is when an atrial tachyarrhythmia is occurring. In DDD mode, the atrial tachyarrhythmia could cause a ventricular tachyarrhythmia to occur (referred to as pacemaker mediated tachycardia (PMT)). Therefore, upon detection of atrial tachyarrhythmia, many ICDs switch modes to VVI or some other mode in which the atrial activity is not used to control the ventricular activity. Many other mode switches are also well known to those skilled in the art, including switching to an antitachycardia pacing mode.
In some instances, devices switch modes due to false indicators of atrial tachyarrhythmias. One such false indicator is far field R-waves, namely ventricular depolarizations that are sensed by atrial sensing circuitry and erroneously believed to be atrial depolarizations. When far-field R-waves are counted as atrial events along with actual atrial events, the ICD will determine that the atrial rate is higher than it actually is, which may cause the ICD to incorrectly believe that an atrial tachyarrhythmia is occurring and to inappropriately switch modes.
It would therefore be advantageous to provide an implantable cardiac stimulation device capable of discriminating between actual atrial events and far-field R-waves.